The polarity of GaN epitaxial layers grown on GaN and AlN buffer layers was investigated and found to be dependent on nitridation temperature over the range of 200-700degreesC. When low temperature (LT), 500degreesC, GaN buffer layers are used, GaN epitaxial layers grown on 200degreesC nitrided sapphire have a higher density of N-polar inversion domains. However, a high density of dislocation pits was observed on GaN epitaxial layers based on AFM morphology when GaN epitaxial layers were grown on LT GaN buffer of 700degreesC nitrided sapphire substrate. With high temperature (HT), 850degreesC, AlN buffer layers, the density of N-polar inversion domains in GaN epitaxial layers depends on the thickness of AlN buffer layer. The structural quality of Ga-polar GaN epitaxial layer is dramatically improved when LT GaN and HT AlN buffer layers are combined with an optimized annealing time. The measured full-widths at half-maximum of (0 0 0 2) symmetric and (10.4) asymmetric reflections are 68 and 246 arcsec, respectively, for 1.0 mum GaN epitaxial layers. The results presented here can be implemented to produce low dislocation density, single Ga-polar GaN epitaxial layers. (C) 2003 Elsevier Science B.V. All rights reserved.